NEW YORK — Welcome back to fashion week parties where no expense is spared except for air-conditioning. “It is so ‘ot,” Roland Mouret said in his French-accented English at the Calvin Klein launch party for the new fragrance, Euphoria, on Thursday night in Chelsea. “‘ot” was the perfect word, since the atmosphere inside the raw space on West 21st Street and 10th Avenue transformed into a nightclub was so stiflingly sweltering that it made everyone not only want to drop their consonants, but whatever else they could to keep cool.

The heat had everyone complaining, continuing one of the major trends of New York Fashion Week: moans about the weather (barely nipping gripes about the traffic). “Thank goodness I dressed appropriately — sort of,” said Lisa Airan as she squeezed through the crowd of models, business types, journalists and the occasional celebrity, such as Ashley Olsen, Claire Danes, Lara Flynn Boyle, Scott Weiland and actors whose faces you knew but whose names you couldn’t quite recall. Airan was referring to her pale, knee-length J. Mendel dress with a plunging neckline. And if she wasn’t cool in that barely there outfit, how could anyone else be? But at least some retained their sense of humor. “Whatever happened to ‘don’t ever let ’em see you sweat?'” joked a damp Men’s Health editor in chief David Zinczenko.

Olsen, meanwhile, maintained her sangfroid as she sat somewhat morosely with friends at a table. “I wasn’t going to go to any shows,” she said, after initially waving off any questions. “But then I decided at the last minute that I wanted to go to Calvin.”

As guests sipped Taittinger champagne and nibbled on Lure Fish Bar’s tiny treats, the vibe seemed right out of a Nineties nightclub, complete with backlit bars, pounding music, crammed-in sweaty bodies, bouncers behind a velvet rope and even cigarette smoking inside. People searched for the coolest spot in the room, gathering with faces raised under a ceiling air conditioning unit that was puffing out air like an exhausted marathoner. Outside, taxis seemed to be on a perennial resupply run, disgorging more cases of water, soft drinks and alcohol every 20 minutes. Guests heaved sighs of relief as they left, but not everyone was quite so happy. Any good nightclub bash requires at least one fracas, and Calvin Klein designer Francisco Costa took it upon himself to calm down an irate man who threatened to cause a scene outside. “What do you want?” Costa asked him kindly. “What can we do?”

“You just can’t treat people like that!” the troublemaker yelled before calming down and going back into the bash — proving it’s more important to be part of the “in” crowd than to keep cool.

Some revelers did flee for more air-conditioned climes, though. Co-host Natalia Vodianova and husband Justin Portman popped over to The Double Seven for pal Sophie Dahl‘s birthday, joining a Brit-heavy crowd including Jacquetta Wheeler and Alice Temperley. Others headed downtown to the Zac Posen party at the SoHo Grand. Swarms of girls there were flirting with Diddy, but he only wanted to talk about how he and the designer first met. “He invited me down to his showroom, we talked and we were cool after that,” said the rap impresario.

Posen ended up partying with celeb pals like Claire Danes (of course), Margherita Missoni (of course) and Tara Reid (huh?). “Tomorrow we’re in sales meetings but tonight I’m just going to have a good time,” the designer said before dancing on any tabletop or chair he could find. Tinsley Mortimer had been trying hard not to have too good of a time all week. “I’ve been good, I haven’t been doing the Bungalow thing,” she said.

After midnight, Posen and his posse — including Nicky Hilton, Bijou Phillips and Alexandra Richards — piled into a few Jaguars and rolled up to Home, the name of which never fails to confound Hilton. “I never know if people are asking me if I’m going home to bed or going to Home the club,” she said.

Selma Blair, who arrived sans husband Ahmet Zappa (he was back at the house), declined the invitation to party until dawn. “I’m having fun, but I’m exhausted,” said the actress, before heading to her real home.

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